Quantum transfer at the push of a button

For the first time, the quantum state of a superconducting qubit was transferred with a coaxial cable to another qubit. (Image: ETH Zurich /M. Pechal, T. Walter, P. Kurpiers)

Now it is possible to transfer quantum information at the push of a button and with high fidelity, between two quantum bits roughly a meter apart, thanks to a new technique demonstrated by ETH Zurich.

The main peculiarity of quantum information technologies, such as quantum computers and quantum cryptography, is the use of quantum bits or «qubits» as the elementary unit of information. Differently, from classical bits, qubits cannot just have the value 0 or 1, but also take on so-called superposition states. On the one hand, this results in the possibility to build extremely powerful computers that make use of those superposition states to perform calculations much more efficiently and faster than classical computers.

Then again, those states are additionally extremely delicate and can’t be transmitted just by utilizing regular systems. The issue is that the condition of a stationary qubit first must be changed into an alleged “flying” qubit, for example, a photon, and after that once again into another stationary qubit. A couple of years back analysts could transmit the quantum state of a particle thusly.

In this study, scientists succeeded in realizing such a transmission also from one superconducting solid-state qubit to another one some distance away. They connected two superconducting qubits using a coaxial cable of the kind that is also used to connect to antenna terminals.

The quantum condition of the main qubit, which is characterized by the number of superconducting electron sets (otherwise called Cooper sets) contained in it, was first exchanged to a microwave photon of a resonator utilizing a precisely controlled microwave pulse. From that resonator, the photon could then fly through the coaxial link to a second resonator, within which microwave pulses, again, exchanged its quantum state onto the second qubit.

Philipp Kurpiers, a Ph.D. student in Wallraff’s lab said, “The important point of our method is that the transmission of the quantum state is deterministic, which means that it works at the push of a button. In some earlier experiments, a transfer of quantum states could already be realized, but that transmission was probabilistic: sometimes it worked, but most of the time it didn’t. A successful transmission could, for instance, be signaled by a “heralding photon”. Whenever the transmission hadn’t worked, one simply tried again. In that way, the effective quantum transmission rate was, of course, strongly reduced.”

Andreas Wallraff said, “Our transmission rate for quantum states is among the highest ever realized, and at 80% our transmission fidelity is very good in the first realization of the protocol. Using their technique, the researchers were also able to create a quantum mechanical entanglement between the qubits as many as 50,000 times per second. The transmission procedure itself took less than a millionth of a second, which means that there is quite a bit of room for improvement in the transmission rate. Quantum mechanical entanglement creates an intimate link between two quantum objects even across large distances, a feature that is used for cryptography or quantum teleportation.”

Scientists further wanted to endeavor to utilize two qubits each as transmitter and beneficiary, which makes ensnarement swapping between the qubit sets conceivable. Such a procedure is helpful for bigger quantum computers, which should be worked in the following couple of years.

So far, they only consist of a handful of qubits, but when trying to build larger computers, already for a few hundred qubits one will have to worry about how to connect them most effectively in order to exploit the advantages of a quantum computer in the best possible way.

Much like clusters of single computers used today, quantum computer modules could then be connected together using Wallraff’s technique. The transmission distance, which is currently about a meter, could certainly be increased. Wallraff and his colleagues recently demonstrated that an extremely cold, and thus superconducting, a cable could transmit photons over distances of several tens of meters with very little loss. Wiring together a quantum computing center, therefore, seems to be quite feasible.

Their results are published in the scientific journal Nature this week.